With the introduction of compact disks, digital wireless telephone networks, and audio delivery over the Internet, digital audio has become commonplace. Engineers use a variety of techniques to control the quality and bitrate of digital audio. To understand these techniques, it helps to understand how audio information is represented in a computer and how humans perceive audio.
I. Representation of Audio Information in a Computer
A computer processes audio information as a series of numbers representing the audio information. For example, a single number can represent an audio sample, which is an amplitude (i.e., loudness) at a particular time. Several factors affect the quality of the audio information, including sample depth, sampling rate, and channel mode.
Sample depth (or precision) indicates the range of numbers used to represent a sample. The more values possible for the sample, the higher the quality because the number can capture more subtle variations in amplitude. For example, an 8-bit sample has 256 possible values, while a 16-bit sample has 65,536 possible values.
The sampling rate (usually measured as the number of samples per second) also affects quality. The higher the sampling rate, the higher the quality because more frequencies of sound can be represented. Some common sampling rates are 8,000, 11,025, 22,050, 32,000, 44,100, 48,000, and 96,000 samples/second.
Mono and stereo are two common channel modes for audio. In mono mode, audio information is present in one channel. In stereo mode, audio information is present in two channels, usually labeled the left and right channels. Other modes with more channels, such as 5-channel surround sound, are also possible. Table 1 shows several formats of audio with different quality levels, along with corresponding raw bitrate costs.
TABLE 1Bitrates for different quality audio informationSample DepthSampling RateRaw BitrateQuality(bits/sample)(samples/second)Mode(bits/second)Internet88,000mono64,000telephonytelephone811,025mono88,200CD audio1644,100stereo1,411,200high quality1648,000stereo1,536,000audio
As Table 1 shows, the cost of high quality audio information such as CD audio is high bitrate. High quality audio information consumes large amounts of computer storage and transmission capacity.
II. Processing Audio Information in a Computer
Many computers and computer networks lack the resources to process raw digital audio. Compression (also called encoding or coding) decreases the cost of storing and transmitting audio information by converting the information into a lower bitrate form. Compression can be lossless (in which quality does not suffer) or lossy (in which quality suffers but bitrate reduction from subsequent lossless compression is more dramatic). Decompression (also called decoding) extracts a reconstructed version of the original information from the compressed form.
A. Standard Perceptual Audio Encoders and Decoders
Generally, the goal of audio compression is to digitally represent audio signals to provide maximum signal quality with the least possible amount of bits. A conventional audio coder/decoder [“codec”] system uses subband/transform coding, quantization, rate control, and variable length coding to achieve its compression. The quantization and other lossy compression techniques introduce potentially audible noise into an audio signal. The audibility of the noise depends on how much noise there is and how much of the noise the listener perceives. The first factor relates mainly to objective quality, while the second factor depends on human perception of sound.
An audio encoder can use various techniques to provide the best possible quality for a given bitrate, including transform coding, modeling human perception of audio, and rate control. As a result of these techniques, an audio signal can be more heavily quantized at selected frequencies or times to decrease bitrate, yet the increased quantization will not significantly degrade perceived quality for a listener.
FIG. 1 shows a generalized diagram of a transform-based, perceptual audio encoder (100) according to the prior art. FIG. 2 shows a generalized diagram of a corresponding audio decoder (200) according to the prior art. Though the codec system shown in FIGS. 1 and 2 is generalized, it has characteristics found in several real world codec systems, including versions of Microsoft Corporation's Windows Media Audio [“WMA”] encoder and decoder, in particular WMA version 8 [“WMA8”]. Other codec systems are provided or specified by the Motion Picture Experts Group, Audio Layer 3 [“MP3”] standard, the Motion Picture Experts Group 2, Advanced Audio Coding [“AAC”] standard, and Dolby AC3. For additional information about these other codec systems, see the respective standards or technical publications.
1. Perceptual Audio Encoder
Overall, the encoder (100) receives a time series of input audio samples (105), compresses the audio samples (105) in one pass, and multiplexes information produced by the various modules of the encoder (100) to output a bitstream (195) at a constant or relatively constant bitrate. The encoder (100) includes a frequency transformer (110), a multi-channel transformer (120), a perception modeler (130), a weighter (140), a quantizer (150), an entropy encoder (160), a controller (170), and a bitstream multiplexer [“MUX”] (180).
The frequency transformer (110) receives the audio samples (105) and converts them into data in the frequency domain. For example, the frequency transformer (110) splits the audio samples (105) into blocks, which can have variable size to allow variable temporal resolution. Small blocks allow for greater preservation of time detail at short but active transition segments in the input audio samples (105), but sacrifice some frequency resolution. In contrast, large blocks have better frequency resolution and worse time resolution, and usually allow for greater compression efficiency at longer and less active segments. Blocks can overlap to reduce perceptible discontinuities between blocks that could otherwise be introduced by later quantization. For multi-channel audio, the frequency transformer (110) uses the same pattern of windows for each channel in a particular frame. The frequency transformer (110) outputs blocks of frequency coefficient data to the multi-channel transformer (120) and outputs side information such as block sizes to the MUX (180).
Transform coding techniques convert information into a form that makes it easier to separate perceptually important information from perceptually unimportant information. The less important information can then be quantized heavily, while the more important information is preserved, so as to provide the best perceived quality for a given bitrate.
For multi-channel audio data, the multiple channels of frequency coefficient data produced by the frequency transformer (110) often correlate. To exploit this correlation, the multi-channel transformer (120) can convert the multiple original, independently coded channels into jointly coded channels. For example, if the input is stereo mode, the multi-channel transformer (120) can convert the left and right channels into sum and difference channels:
                                                        X              Sum                        ⁡                          [              k              ]                                =                                                                      X                  Left                                ⁡                                  [                  k                  ]                                            +                                                X                  Right                                ⁡                                  [                  k                  ]                                                      2                          ,        and                            (        1        )                                                      X            Diff                    ⁡                      [            k            ]                          =                                                                              X                  Left                                ⁡                                  [                  k                  ]                                            -                                                X                  Right                                ⁡                                  [                  k                  ]                                                      2                    .                                    (        2        )            Or, the multi-channel transformer (120) can pass the left and right channels through as independently coded channels. The decision to use independently or jointly coded channels is predetermined or made adaptively during encoding. For example, the encoder (100) determines whether to code stereo channels jointly or independently with an open loop selection decision that considers the (a) energy separation between coding channels with and without the multi-channel transform and (b) the disparity in excitation patterns between the left and right input channels. Such a decision can be made on a window-by-window basis or only once per frame to simplify the decision. The multi-channel transformer (120) produces side information to the MUX (180) indicating the channel mode used.
The encoder (100) can apply multi-channel rematrixing to a block of audio data after a multi-channel transform. For low bitrate, multi-channel audio data in jointly coded channels, the encoder (100) selectively suppresses information in certain channels (e.g., the difference channel) to improve the quality of the remaining channel(s) (e.g., the sum channel). For example, the encoder (100) scales the difference channel by a scaling factor ρ:{tilde over (X)}Diff[k]=ρ·XDiff[k],   (3)where the value of ρ is based on: (a) current average levels of a perceptual audio quality measure such as Noise to Excitation Ratio [“NER”], (b) current fullness of a virtual buffer, (c) bitrate and sampling rate settings of the encoder (100), and (d) the channel separation in the left and right input channels.
The perception modeler (130) processes audio data according to a model of the human auditory system to improve the perceived quality of the reconstructed audio signal for a given bitrate. For example, an auditory model typically considers the range of human hearing and critical bands. The human nervous system integrates sub-ranges of frequencies. For this reason, an auditory model may organize and process audio information by critical bands. Different auditory models use a different number of critical bands (e.g., 25, 32, 55, or 109) and/or different cut-off frequencies for the critical bands. Bark bands are a well-known example of critical bands. Aside from range and critical bands, interactions between audio signals can dramatically affect perception. An audio signal that is clearly audible if presented alone can be completely inaudible in the presence of another audio signal, called the masker or the masking signal. The human ear is relatively insensitive to distortion or other loss in fidelity (i.e., noise) in the masked signal, so the masked signal can include more distortion without degrading perceived audio quality. In addition, an auditory model can consider a variety of other factors relating to physical or neural aspects of human perception of sound.
Using an auditory model, an audio encoder can determine which parts of an audio signal can be heavily quantized without introducing audible distortion, and which parts should be quantized lightly or not at all. Thus, the encoder can spread distortion across the signal so as to decrease the audibility of the distortion. The perception modeler (130) outputs information that the weighter (140) uses to shape noise in the audio data to reduce the audibility of the noise. For example, using any of various techniques, the weighter (140) generates weighting factors (sometimes called scaling factors) for quantization matrices (sometimes called masks) based upon the received information. The weighting factors in a quantization matrix include a weight for each of multiple quantization bands in the audio data, where the quantization bands are frequency ranges of frequency coefficients. The number of quantization bands can be the same as or less than the number of critical bands. Thus, the weighting factors indicate proportions at which noise is spread across the quantization bands, with the goal of minimizing the audibility of the noise by putting more noise in bands where it is less audible, and vice versa. The weighting factors can vary in amplitudes and number of quantization bands from block to block. The weighter (140) then applies the weighting factors to the data received from the multi-channel transformer (120).
In one implementation, the weighter (140) generates a set of weighting factors for each window of each channel of multi-channel audio, or shares a single set of weighting factors for parallel windows of jointly coded channels. The weighter (140) outputs weighted blocks of coefficient data to the quantizer (150) and outputs side information such as the sets of weighting factors to the MUX (180).
A set of weighting factors can be compressed for more efficient representation using direct compression. In the direct compression technique, the encoder (100) uniformly quantizes each element of a quantization matrix. The encoder then differentially codes the quantized elements, and Huffman codes the differentially coded elements. In some cases (e.g., when all of the coefficients of particular quantization bands have been quantized or truncated to a value of 0), the decoder (200) does not require weighting factors for all quantization bands. In such cases, the encoder (100) gives values to one or more unneeded weighting factors that are identical to the value of the next needed weighting factor in a series, which makes differential coding of elements of the quantization matrix more efficient.
Or, for low bitrate applications, the encoder (100) can parametrically compress a quantization matrix to represent the quantization matrix as a set of parameters, for example, using Linear Predictive Coding [“LPC”] of pseudo-autocorrelation parameters computed from the quantization matrix.
The quantizer (150) quantizes the output of the weighter (140), producing quantized coefficient data to the entropy encoder (160) and side information including quantization step size to the MUX (180). Quantization maps ranges of input values to single values. In a generalized example, with uniform, scalar quantization by a factor of 3.0, a sample with a value anywhere between −1.5 and 1.499 is mapped to 0, a sample with a value anywhere between 1.5 and 4.499 is mapped to 1, etc. To reconstruct the sample, the quantized value is multiplied by the quantization factor, but the reconstruction is imprecise. Continuing the example started above, the quantized value 1 reconstructs to 1×3=3; it is impossible to determine where the original sample value was in the range 1.5 to 4.499. Quantization causes a loss in fidelity of the reconstructed value compared to the original value, but can dramatically improve the effectiveness of subsequent lossless compression, thereby reducing bitrate. Adjusting quantization allows the encoder (100) to regulate the quality and bitrate of the output bitstream (195) in conjunction with the controller (170). In FIG. 1, the quantizer (150) is an adaptive, uniform, scalar quantizer. The quantizer (150) applies the same quantization step size to each frequency coefficient, but the quantization step size itself can change from one iteration of a quantization loop to the next to affect quality and the bitrate of the entropy encoder (160) output. Other kinds of quantization are non-uniform quantization, vector quantization, and/or non-adaptive quantization.
The entropy encoder (160) losslessly compresses quantized coefficient data received from the quantizer (150). The entropy encoder (160) can compute the number of bits spent encoding audio information and pass this information to the rate/quality controller (170).
The controller (170) works with the quantizer (150) to regulate the bitrate and/or quality of the output of the encoder (100). The controller (170) receives information from other modules of the encoder (100) and processes the received information to determine a desired quantization step size given current conditions. The controller (170) outputs the quantization step size to the quantizer (150) with the goal of satisfying bitrate and quality constraints. U.S. patent application Ser. No. 10/017,694, filed Dec. 14, 2001, entitled “Quality and Rate Control Strategy for Digital Audio,” published on Jun. 19, 2003, as Publication No. US-2003-0115050-A1, includes description of quality and rate control as implemented in an audio encoder of WMA8, as well as additional description of other quality and rate control techniques.
The encoder (100) can apply noise substitution and/or band truncation to a block of audio data. At low and mid-bitrates, the audio encoder (100) can use noise substitution to convey information in certain bands. In band truncation, if the measured quality for a block indicates poor quality, the encoder (100) can completely eliminate the coefficients in certain (usually higher frequency) bands to improve the overall quality in the remaining bands.
The MUX (180) multiplexes the side information received from the other modules of the audio encoder (100) along with the entropy encoded data received from the entropy encoder (160). The MUX (180) outputs the information in a format that an audio decoder recognizes. The MUX (180) includes a virtual buffer that stores the bitstream (195) to be output by the encoder (100).
2. Perceptual Audio Decoder
Overall, the decoder (200) receives a bitstream (205) of compressed audio information including entropy encoded data as well as side information, from which the decoder (200) reconstructs audio samples (295). The audio decoder (200) includes a bitstream demultiplexer [“DEMUX”] (210), an entropy decoder (220), an inverse quantizer (230), a noise generator (240), an inverse weighter (250), an inverse multi-channel transformer (260), and an inverse frequency transformer (270).
The DEMUX (210) parses information in the bitstream (205) and sends information to the modules of the decoder (200). The DEMUX (210) includes one or more buffers to compensate for variations in bitrate due to fluctuations in complexity of the audio, network jitter, and/or other factors.
The entropy decoder (220) losslessly decompresses entropy codes received from the DEMUX (210), producing quantized frequency coefficient data. The entropy decoder (220) typically applies the inverse of the entropy encoding technique used in the encoder.
The inverse quantizer (230) receives a quantization step size from the DEMUX (210) and receives quantized frequency coefficient data from the entropy decoder (220). The inverse quantizer (230) applies the quantization step size to the quantized frequency coefficient data to partially reconstruct the frequency coefficient data.
From the DEMUX (210), the noise generator (240) receives information indicating which bands in a block of data are noise substituted as well as any parameters for the form of the noise. The noise generator (240) generates the patterns for the indicated bands, and passes the information to the inverse weighter (250).
The inverse weighter (250) receives the weighting factors from the DEMUX (210), patterns for any noise-substituted bands from the noise generator (240), and the partially reconstructed frequency coefficient data from the inverse quantizer (230). As necessary, the inverse weighter (250) decompresses the weighting factors, for example, entropy decoding, inverse differentially coding, and inverse quantizing the elements of the quantization matrix. The inverse weighter (250) applies the weighting factors to the partially reconstructed frequency coefficient data for bands that have not been noise substituted. The inverse weighter (250) then adds in the noise patterns received from the noise generator (240) for the noise-substituted bands.
The inverse multi-channel transformer (260) receives the reconstructed frequency coefficient data from the inverse weighter (250) and channel mode information from the DEMUX (210). If multi-channel audio is in independently coded channels, the inverse multi-channel transformer (260) passes the channels through. If multi-channel data is in jointly coded channels, the inverse multi-channel transformer (260) converts the data into independently coded channels.
The inverse frequency transformer (270) receives the frequency coefficient data output by the multi-channel transformer (260) as well as side information such as block sizes from the DEMUX (210). The inverse frequency transformer (270) applies the inverse of the frequency transform used in the encoder and outputs blocks of reconstructed audio samples (295).
III. Controlling Rate and Quality of Audio Information
Different audio applications have different quality and bitrate requirements. Certain applications require constant or relatively constant bitrate [“CBR”]. One such CBR application is encoding audio for streaming over the Internet. Other applications require constant or relatively constant quality over time for compressed audio information, resulting in variable bitrate [“VBR”] output.
The goal of a CBR encoder is to output compressed audio information at a constant bitrate despite changes in the complexity of the audio information. Complex audio information is typically less compressible than simple audio information. To meet bitrate requirements, the CBR encoder can adjust how the audio information is quantized. The quality of the compressed audio information then varies, with lower quality for periods of complex audio information due to increased quantization and higher quality for periods of simple audio information due to decreased quantization.
While adjustment of quantization and audio quality is necessary at times to satisfy CBR requirements, some CBR encoders can cause unnecessary changes in quality, which can result in thrashing between high quality and low quality around the appropriate, middle quality. Moreover, when changes in audio quality are necessary, some CBR encoders often cause abrupt changes, which are more noticeable and objectionable than smooth changes.
WMA version 7.0 [“WMA7”] includes an audio encoder that can be used for CBR encoding of audio information for streaming. The WMA7 encoder uses a virtual buffer and rate control to handle variations in bitrate due to changes in the complexity of audio information. In general, the WMA7 encoder uses one-pass CBR rate control. In a one-pass encoding scheme, an encoder analyzes the input signal and generates a compressed bit stream in the same pass through the input signal.
To handle short-term fluctuations around the constant bitrate (such as those due to brief variations in complexity), the WMA7 encoder uses a virtual buffer that stores some duration of compressed audio information. For example, the virtual buffer stores compressed audio information for 5 seconds of audio playback. The virtual buffer outputs the compressed audio information at the constant bitrate, so long as the virtual buffer does not underflow or overflow. Using the virtual buffer, the encoder can compress audio information at relatively constant quality despite variations in complexity, so long as the virtual buffer is long enough to smooth out the variations. In practice, virtual buffers must be limited in duration in order to limit system delay, however, and buffer underflow or overflow can occur unless the encoder intervenes.
To handle longer-term deviations from the constant bitrate (such as those due to extended periods of complexity or silence), the WMA7 encoder adjusts the quantization step size of a uniform, scalar quantizer in a rate control loop. The relation between quantization step size and bitrate is complex and hard to predict in advance, so the encoder tries one or more different quantization step sizes until the encoder finds one that results in compressed audio information with a bitrate sufficiently close to a target bitrate. The encoder sets the target bitrate to reach a desired buffer fullness, preventing buffer underflow and overflow. Based upon the complexity of the audio information, the encoder can also allocate additional bits for a block or deallocate bits when setting the target bitrate for the rate control loop.
The WMA7 encoder measures the quality of the reconstructed audio information for certain operations (e.g., deciding which bands to truncate). The WMA7 encoder does not use the quality measurement in conjunction with adjustment of the quantization step size in a quantization loop, however.
The WMA7 encoder controls bitrate and provides good quality for a given bitrate, but can cause unnecessary quality changes. Moreover, with the WMA7 encoder, necessary changes in audio quality are not as smooth as they could be in transitions from one level of quality to another.
U.S. patent application Ser. No. 10/017,694 includes description of quality and rate control as implemented in the WMA8 encoder, as well as additional description of other quality and rate control techniques. In general, the WMA8 encoder uses one-pass CBR quality and rate control, with complexity estimation of future frames. For additional detail, see U.S. patent application Ser. No. 10/017,694.
The WMA8 encoder smoothly controls rate and quality, and provides good quality for a given bitrate. As a one-pass encoder, however, the WMA8 encoder relies on partial and incomplete information about future frames in an audio sequence.
Numerous other audio encoders use rate control strategies. For example, see U.S. Pat. No. 5,845,243 to Smart et al. Such rate control strategies potentially consider information other than or in addition to current buffer fullness, for example, the complexity of the audio information.
Several international standards describe audio encoders that incorporate distortion and rate control. The MP3 and AAC standards each describe techniques for controlling distortion and bitrate of compressed audio information.
In MP3, the encoder uses nested quantization loops to control distortion and bitrate for a block of audio information called a granule. Within an outer quantization loop for controlling distortion, the MP3 encoder calls an inner quantization loop for controlling bitrate.
In the outer quantization loop, the MP3 encoder compares distortions for scale factor bands to allowed distortion thresholds for the scale factor bands. A scale factor band is a range of frequency coefficients for which the encoder calculates a weight called a scale factor. Each scale factor starts with a minimum weight for a scale factor band. After an iteration of the inner quantization loop, the encoder amplifies the scale factors until the distortion in each scale factor band is less than the allowed distortion threshold for that scale factor band, with the encoder calling the inner quantization loop for each set of scale factors. In special cases, the encoder exits the outer quantization loop even if distortion exceeds the allowed distortion threshold for a scale factor band (e.g., if all scale factors have been amplified or if a scale factor has reached a maximum amplification).
In the inner quantization loop, the MP3 encoder finds a satisfactory quantization step size for a given set of scale factors. The encoder starts with a quantization step size expected to yield more than the number of available bits for the granule. The encoder then gradually increases the quantization step size until it finds one that yields fewer than the number of available bits.
The MP3 encoder calculates the number of available bits for the granule based upon the average number of bits per granule, the number of bits in a bit reservoir, and an estimate of complexity of the granule called perceptual entropy. The bit reservoir counts unused bits from previous granules. If a granule uses less than the number of available bits, the MP3 encoder adds the unused bits to the bit reservoir. When the bit reservoir gets too full, the MP3 encoder preemptively allocates more bits to granules or adds padding bits to the compressed audio information. The MP3 encoder uses a psychoacoustic model to calculate the perceptual entropy of the granule based upon the energy, distortion thresholds, and widths for frequency ranges called threshold calculation partitions. Based upon the perceptual entropy, the encoder can allocate more than the average number of bits to a granule.
For additional information about MP3 and AAC, see the MP3 standard (“ISO/IEC 11172-3, Information Technology—Coding of Moving Pictures and Associated Audio for Digital Storage Media at Up to About 1.5 Mbit/s—Part 3: Audio”) and the AAC standard.
Other audio encoders use a combination of filtering and zero tree coding to jointly control quality and bitrate, in which an audio encoder decomposes an audio signal into bands at different frequencies and temporal resolutions. The encoder formats band information such that information for less perceptually important bands can be incrementally removed from a bitstream, if necessary, while preserving the most information possible for a given bitrate. For more information about zero tree coding, see Srinivasan et al., “High-Quality Audio Compression Using an Adaptive Wavelet Packet Decomposition and Psychoacoustic Modeling,” IEEE Transactions on Signal Processing, Vol. 46, No. 4, pp. (April 1998).
Outside of the field of audio encoding, various joint quality and bitrate control strategies for video encoding have been published. For example, see U.S. Pat. No. 5,686,964 to Naveen et al.; U.S. Pat. No. 5,995,151 to Naveen et al.; Caetano et al., “Rate Control Strategy for Embedded Wavelet Video Coders,” IEEE Electronics Letters, pp 1815-17 (Oct. 14, 1999); Ribas-Corbera et al., “Rate Control in DCT Video Coding for Low-Delay Communications,” IEEE Trans Circuits and Systems for Video Tech., Vol. 9, No 1, (February 1999); and Westerink et al., “Two-pass MPEG-2 Variable Bit Rate Encoding,” IBM Journal of Res. Dev., Vol. 43, No. 4 (July 1999).
The Westerink article describes a two-pass VBR control strategy for video compression. As such, the control strategy described therein cannot be simply applied to other types of media such as audio. For one thing, the video input in the Westerink article is partitioned at regular times into uniformly sized video frames. The Westerink article does not describe how to perform two-pass VBR control for media with variable-size encoding units. Also, for video coding, there are reasonable models relating quantization step size to quality and step size to bits, as used in the Westerink article. These models cannot be simply applied to audio data in many cases, however, due to the erratic step-rate-distortion performance of audio data.
As one might expect given the importance of quality and rate control to encoder performance, the fields of quality and rate control are well developed. Whatever the advantages of previous quality and rate control strategies, however, they do not offer the performance advantages of the present invention.